Mattermost does have an Github Repository with a choice of three licenses: MIT (if using versions compiled by them), AGPLv3 (if compiled by you) or an Enterprise license. I would count that as open source.

A more general business management application like Odoo could work?

Neat idea. This could be refined by adding a git hook that runs (rip)grep on the entire codebase and fails if anything is found upon commit may accomplish a similar result and stop the code from being committed entirely. Requires a bit more setup work on de developers end, though.

KeepassXC is multiplatform. (Also: KeepassDX is quite nice as an an Android app for Keepass databases.)